Epilepsy is a major neurological disorder with significant economic and human burdens. One of the most common forms is mesial temporal lobe epilepsy (TLE). Unfortunately, medical treatment of TLE fails in many cases, leaving a large unmet clinical need. Therefore, a thorough understanding of the neuronal circuits involved in seizure initiation and propagation will drive the development of novel therapies. A hallmark of mesial temporal lobe epilepsy for many patients is hippocampal sclerosis. The hippocampus is particularly susceptible to excitotoxicity. Key sites of neuronal death are the hilus of the dentat gyrus, where many inhibitory GABAergic interneurons are lost. Recurrent seizures trigger an increase in both neurogenesis and the development of remaining neurons. This is particularly true for the dentate gyrus, whose responses include: birth and migration of new granule cells, appearance of novel basal dendrites, and an increase in their axonal projections including excitatory collaterals back onto neighboring granule cells (mossy fiber sprouting). Which of all these responses is most responsible for the development of spontaneous seizures? This proposal will test the hypothesis that reduced activity of inhibitory interneurons is a key driver f epileptogenesis. This hypothesis was developed to explain the serendipitous finding that expression of a modified leak K+ channel (TREK-M) in rat dentate hilar neurons triggered limbic seizures similar to those that occur in TLE patients. The research will use a novel chemogenetic approach based on adeno-associated viral delivery of TREK-M whose expression is dependent on the action of Cre recombinase and regulated by doxycycline. This allows us to exploit Cre-driver mice that have been developed as a result of the NIH Neuroscience Blueprint. The ability of chemogenetic silencing of GABAergic neurons will be first tested in mice where Cre is expressed in all GABAergic interneurons. The first aim will validate delivery, record seizure activity, and examine the pathology that results, focusing on mossy fiber sprouting. The second aim will begin to dissect which of the many GABAergic subtypes are critical for seizure generation. This innovative chemogenetic approach will likely have a major impact on the field of neuroscience, as it provides a longer time scale to probe complex behaviors than is possible with optogenetics. The studies may also provide the first animal model of spontaneous recurrent seizures without neuronal death, which would mimic human TLE patients who do not show hippocampal sclerosis.